Optical trap catches atoms swinging in time to theory

Trapped, cold atoms swing together in a perfect match with theory.

It's bizarre to feel awestruck and disappointed at the same time. Yet this is often how I feel when I read articles about ultracold atoms and Bose Einstein condensates. I'll get to the awesome and awestruck parts later, but let me explain my disappointment. These experiments sit right at the boundary between classical and quantum physics. When we play with ultracold atoms, we make macroscopic objects do quantum things. And what have we discovered? That quantum mechanics is pretty much correct.

So, when the latestPhysical Review Letter on the motion of ultracold atoms came out, I was a little underwhelmed. And truth be told, the results are not surprising. Nevertheless, there is one notable thing in the research, common to a lot of stuff in this field: the results are just plain beautiful.

Show me the ultracold atom fridge, please

Let's start with a quick overview of the experiment. The researchers have a supply of ultracold atoms (at a temperature of a couple microKelvin) that they place between two mirrors. They shine a laser on the mirrors and atoms. The light from the laser gets reflected back and forth between the mirrors, creating a fixed pattern. In certain places in the optical cavity the light has no intensity, while in others it is very bright.

Even though the lasers don't have the right color to be absorbed by the atoms, the atoms still feel the light through the way the light fields push their electrons about. This produces a net force on the atoms, trapping them in the places where the light has no intensity. The atoms end up divided among a few of these little traps and, when they move in the trap, they end up swinging back and forth, as if they were sitting on a swing.

If this was truly a quantum system, the motion in the trap should have a series of fixed energy levels. In other words, the collective motion of the atoms should occur at a fixed set of frequencies, and the atoms should all oscillate together. The researchers wanted to see if this was actually the case.

To do this, they had to shine a second laser with a different wavelength into the optical cavity. The different wavelength ensures that this laser has intensity peaks where the atoms are trapped. The laser light is absorbed by the atoms in a very peculiar fashion. The atoms can absorb a photon of light, but the light is the wrong color, so it can't keep it. Instead, it can strip a little bit of energy from the photon to use for some other process, and then release a photon with slightly less energy—this photon has a longer wavelength and is called the Stokes photon.

Alternatively, the atoms can give up some energy to the absorbed photon, resulting in the atom emitting a photon with a shorter wavelength, called an anti-Stokes photon. In the case of the atoms trapped in the light field, there is only one process that the energy for these photons can be diverted to: trap motion.

To summarize: the trap acts like a swing, so no matter what, the atoms must vibrate back and forth at a set of fixed frequencies. But unless they all vibrate together, the Stokes or anti-Stokes light fields will never build up to levels that allow us to detect them. In addition, if the ultracold atoms are, well, ultracold, they won't be moving much at all, so we should initially only see Stokes light. Only after the atoms climb up the ladder of frequencies should we see an increasing amount of anti-Stokes light.

This is indeed what the researchers saw. The atoms do move collectively, although the collective motion of the atoms in one trap becomes decoupled from the motion of adjacent traps quite quickly. Which, I have to admit, I was not surprised by. In fact, I would have been more surprised if they had observed that the adjacent traps remained strongly coupled to each other, or that no collective motion was observed.

Beauty in having no freedom?

So why am I awestruck? Simple. In most of physics—and, indeed, in all of science—we rely on theory to understand our results. We build mathematical models and fit them to our results. But usually there are some parts of the system that remain unknown, or are too complicated to model, so we approximate the parts we can't model and leave the unknown parts as free parameters. Then, when the results don't fit, we vary the free parameters to make them fit.

We don't do that arbitrarily; we have some idea of what these parameters should be and what they cannot be. And we have other experiments to provide a sanity check, so this isn't a bad thing we are talking about here. But it is... unsatisfactory. The good people doing work with ultracold atoms don't have this problem. They know every value that needs to go into their models. No free parameters, no guess work, and no excuses. If the model doesn't fit, there is nothing to play with to save you.

And the amazing thing is that they do fit. Experiment and theory in its purest form, overlapping one another beautifully. That is what makes the work on ultracold atoms awesome.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.